Multi-electrode stimulation and recording in the isolated retina

Abstract

As part of an exploration of the feasibility of an epi-retinal prosthesis, we developed an experimental method to electrically stimulate and record from retinal neurons using a micro-fabricated multi-electrode array. An isolated retina is placed on an array of 10 μm diameter disk electrodes with the ganglion cell side of the retina facing the electrode surfaces. The retina is bathed in oxygenated Ames’ medium and warmed in order to sustain it in vitro for the duration of an experiment, typically 4–9 h. To reduce stimulus artifacts, the electrodes are grouped into two clusters — one used for stimulation and the other for recording — spaced several hundred microns apart, and electrodes are insulated with both silicon nitride and a 10 μm thick layer of polyimide. Stimuli are delivered to the array using an optically isolated current source stimulator, and the resulting responses recorded with an eight channel nerve response amplifier. Stimulation and recording are performed under computer control. A variety of physiologic measurements is described in order to illustrate the strengths and drawbacks of this method.

Introduction

The last decade has seen increasing interest in the development of retina-based visual prostheses which might someday restore partial vision to patients blinded by photoreceptor diseases like retinitis pigmentosa and macular degeneration (Chow and Chow, 1997, Eckmiller, 1997, Rizzo and Wyatt, 1997, Humayun et al., 1999, Zrenner et al., 1999). The long-term success of such devices will be tied to our understanding of how retinal neurons and networks are activated when extracellular electric stimuli are applied to the retinal surface.

There is a substantial and diverse literature devoted to electric stimulation of the retina. Current research is directed at retinal prosthesis development while earlier efforts were aimed at understanding fundamental retinal processing using a novel type of input. In the majority of these studies, electric stimuli were delivered through conventional electrodes having one or two conductors at the end of an elongated probe. Electrodes of this type offer a fairly limited choice of active stimulating geometry — typically a central point or disk, optionally surrounded by a concentric ring. Furthermore, they are not well-suited to measuring a neuron's excitation thresholds at a number of different stimulating electrode positions. When our group performed such experiments (Jensen et al., 1996, Rizzo et al., 1997), we found it necessary to raise and re-lower the stimulating electrode between measurements in order to reduce the likelihood of dragging the retina. Dragging was undesirable because it could introduce electrode placement imprecision or “loss” of a neuron which had been inadvertently moved away from the recording electrode. The possibility of dragging could not be completely eliminated, however, since the stimulating electrode had to be physically moved between threshold measurements.

Mechanical disruption of the retina may be avoided by using a multi-electrode array instead of a conventional electrode (Kuras and Gutmanien≐, 1997, Greenberg, 1998). Stimulus positions and geometries are controlled by the choice of stimulator connections to the array, allowing for rapid switching among a large number of active configurations. Photolithographic techniques, which have been used in electrophysiology research since the 1970s (Pickard, 1979), make it possible to pattern these arrays with essentially arbitrary electrode geometries and distributions. Application of electrode arrays to the study of retina, moreover, has recently been demonstrated in amphibians and mammals (Meister et al., 1994, DeVries and Baylor, 1997).

This paper describes a method to stimulate and record from neurons in isolated retinas using a planar, photo-lithographically patterned multi-electrode array. To demonstrate a practical application of the method, results from a number of physiologic measurements are presented and discussed. These also motivate a discussion of the method's strengths and drawbacks.